Commitments and Contingencies - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015 Loan	Mar. 31, 2014
Commitments and Contingencies [Line Items]
Rental expense	$ 1,400,000	$ 700,000
Annual lease payments	1,200,000	600,000
Security deposit made under lease agreement	600,000
Security deposit made under lease agreement, cash	200,000
Unfunded loan balance	1,900,000
Number of unfunded loans	192
Date of fully funded loans	Apr. 03, 2015
Remaining limit of contingent loan purchase commitment	53,900,000
Maximum cash pledged	5,000,000
Pledged and restricted to support contingent obligation	3,400,000
Contingent loan purchase commitment limit	$ 3,600,000
San Francisco
Commitments and Contingencies [Line Items]
Area of property (square feet)	141,000
Lease agreement expiration date	Jun. 01, 2022
Lease agreement renewal term	5 years
Westborough
Commitments and Contingencies [Line Items]
Area of property (square feet)	20,000